U.S. Pat. No. 4,686,437 to Langley et al. entitled Electromechanical Energy Conversion System describes a motor control system whereby a microprocessor or similar electronics can be used to control electric motors. The sensitive electronics are coupled to the motor via a power transistor switching stage wherein the transistors operate in on/off fashion to control the current flow to the motor windings in accord with control signals from the microprocessor. The microprocessor receives feedback from the motor indicating rotor position and other controllable parameters. The microprocessor may also receive command signals usable for servo control. The command signals and the feedback signals are compared to determine the frequency, amplitude and phase for the winding energization, and the rotor position feedback is used to determine the proper commutation of the winding excitation. The microprocessor, either alone or with other components, provides on/off control signals to the power stages which, in turn, supply current to the windings with the desired amplitude, frequency and phase commutated according to the rotor position. Most present day servo motor control system utilize this basic control system.
U.S. Pat. Nos. 4,447,771 and 4,490,661 to Whited and Brown et al., respectively, disclose motor control systems in which digital electronics control power switching transistors via a pulse width modulator (PWM) so that the excitation current to the motor is a function of the pulse width. Both Whited and Brown et. al. disclose techniques for controlling the phase angle of the excitation current relative to the rotor position to improve the torque efficiency of the system.
Most present day systems use pulse width modulation as disclosed by Whited and Brown et al. to convert the control signals from the sensitive electronics to the desired excitation currents for the motor windings. In such systems used to control large motors isolation is necessary between the sensitive electronic controller and the brute force power stages that drive the motor. Optical coupling is used to provide a high degree of isolation in most present day controller designs which operate at a PWM switching rate of about 2 KHz.
There is an ever present desire to provide smaller and less costly controllers and controllers with high overall system efficiency. The invention provides those advantages for any size motor. However, the advantages are more pronounced for large motors in the range of 10 to 150 horsepower and are most pronounced for the very large motors in the 150 to 500 horsepower range. In the higher horsepower ranges inefficient use of components and inefficient overall system design directly translate into high controller costs and high usage costs.
One approach to smaller and less costly controllers is to increase the switching frequency of the PWM power stage. In theory at least, higher switching frequencies should make it possible to control the needed power for large motors using smaller and less expensive components. However, the problems associated with operating the power stages at higher switching frequencies have proven to be either insolvable or prohibitively costly both in space and money. High frequency noise can bleed off power from the system and seriously affect the system efficiency. Noise of the common mode variety (noise that goes up and down together on a parallel conductor pair), can be particularly difficult to detect and even more difficult to eliminate.
An object of the present invention is to provide improved isolation between the sensitive electronic stages and the associated power stages.
Another object of the present invention is to provide effective isolation coupling between the electronic stages and the associated power stages in a motor control system where the power stage can operate at a switching rate in the range of above 6,000 Hz.
Still another object of the invention is to provide an isolation transformer for a motor controller operating at a high power switching rate in the range above 6 KHz with little high frequency power loss and negligible noise generation.
Yet another object is to provide a pulse transformer for a motor controller with higher system efficiency, i.e. maximum torque, relative to energy applied to the controller.
Optical coupling, which provides effective coupling isolation at low switching rates, for example, about 2,000 Hertz, has too much time delay for effective operation in the range above 6,000 Hz. Furthermore, switching at the 6,000 Hz rate can generate transients as high as 100 MHz which are not effectively decoupled with the optical devices. Known high frequency, electronic transformer coupling is too expensive and complicated for effective use in a motor controller.
The pulse transformer in the form of a dynamic coupling inductance according to the invention includes spaced apart primary and secondary windings wound around a common high permeability core. The flux return path for flux through the core is configured to surround the windings. The core is configured so that the core is saturable in normal operation. The windings are spaced as far apart as possible to minimize the inter-winding capacitance. The windings are wound using a small number of turns and small views wires to minimize the surface area of the winding to thereby reduce winding to shell and winding to winding capacitance.
If a rectangular pulse is applied to the primary winding of the pulse transformer, a positive spike appears at the secondary winding corresponding to the rise time and a negative spike appears corresponding to the fall time. The rise time of the positive spike occurs while the core is being driven into saturation and the winding are coupled to one another. Once saturation is reached the windings are decoupled. A similar operation takes place during the fall time. By careful design according to the concept of the invention, turn on and turn off times of pulses passing through the pulse transformer are as little as 100 nanoseconds which is about 10 times faster than could be accomplished using optical coupling.
Rectangular pulses from a microprocessor and other electronic stages pass through the saturable pulse transformer as a positive and a negative spike pair which is supplied to a Schmitt trigger that reconstitutes the rectangular pulse for control of the power transistors. With the arrangement according to the invention only the leading and trailing edges of the pulses pass through the pulse transformer. As a result power stage switching can be achieved at the rate of 10 A/100 nanoseconds. Using conventional components, switching for large motor control systems can be on the order of about 250 nanoseconds to about 450 nanoseconds, depending on the motor size. Faster switching speeds, as fast as 150 nanoseconds, can be achieved by using special high speed components.
The coupling between the electronic and power stages according to the invention results in a very high rejection of common mode noise and elimination of the related energy absorption. Contrary to conventional design, the primary and secondary windings are located on a ferrite core and separated as far as possible to minimize inter-winding capacitance. A minimum number of winding turns and a small diameter wire are used for the winding to minimize the conductive surface area. Although this structure provides relatively poor magnetic coupling between the windings, it is sufficient since only the leading and trailing edges of the pulses actually pass through the transformer.